Boron delivery with liposomes for boron neutron capture therapy (BNCT): biodistribution studies in an experimental model of oral cancer demonstrating therapeutic potential

Boron neutron capture therapy (BNCT) combines selective accumulation of (10)B carriers in tumor tissue with subsequent neutron irradiation. We previously demonstrated the therapeutic efficacy of BNCT in the hamster cheek pouch oral cancer model. Optimization of BNCT depends largely on improving boron targeting to tumor cells. Seeking to maximize the potential of BNCT for the treatment for head and neck cancer, the aim of the present study was to perform boron biodistribution studies in the oral cancer model employing two different liposome formulations that were previously tested for a different pathology, i.e., in experimental mammary carcinoma in BALB/c mice: (1) MAC: liposomes incorporating K[nido-7-CH(3)(CH(2))(15)-7,8-C(2)B(9)H(11)] in the bilayer membrane and encapsulating a hypertonic buffer, administered intravenously at 6 mg B per kg body weight, and (2) MAC-TAC: liposomes incorporating K[nido-7-CH(3)(CH(2))(15)-7,8-C(2)B(9)H(11)] in the bilayer membrane and encapsulating a concentrated aqueous solution of the hydrophilic species Na(3) [ae-B(20)H(17)NH(3)], administered intravenously at 18 mg B per kg body weight. Samples of tumor, precancerous and normal pouch tissue, spleen, liver, kidney, and blood were taken at different times post-administration and processed to measure boron content by inductively coupled plasma mass spectrometry. No ostensible clinical toxic effects were observed with the selected formulations. Both MAC and MAC-TAC delivered boron selectively to tumor tissue. Absolute tumor values for MAC-TAC peaked to 66.6 ± 16.1 ppm at 48 h and to 43.9 ± 17.6 ppm at 54 h with very favorable ratios of tumor boron relative to precancerous and normal tissue, making these protocols particularly worthy of radiobiological assessment. Boron concentration values obtained would result in therapeutic BNCT doses in tumor without exceeding radiotolerance in precancerous/normal tissue at the thermal neutron facility at RA-3.     

Boron neutron capture therapy (BNCT) for liver metastasis: therapeutic efficacy in an experimental model

Boron neutron capture therapy (BNCT) was proposed for untreatable colorectal liver metastases. The present study evaluates tumor control and potential radiotoxicity of BNCT in an experimental model of liver metastasis. BDIX rats were inoculated with syngeneic colon cancer cells DHD/K12/TRb. Tumor-bearing animals were divided into three groups: BPA-BNCT, boronophenylalanine (BPA) + neutron irradiation; Beam only, neutron irradiation; Sham, matched manipulation. The total absorbed dose administered with BPA-BNCT was 13 ± 3 Gy in tumor and 9 ± 2 Gy in healthy liver. Three weeks post-treatment, the tumor surface area post-treatment/pre-treatment ratio was 0.46 ± 0.20 for BPA-BNCT, 2.7 ± 1.8 for Beam only and 4.5 ± 3.1 for Sham. The pre-treatment tumor nodule mass of 48 ± 19 mg fell significantly to 19 ± 16 mg for BPA-BNCT, but rose significantly to 140 ± 106 mg for Beam only and to 346 ± 302 mg for Sham. For both end points, the differences between the BPA-BNCT group and each of the other groups were statistically significant (ANOVA). No clinical, macroscopic or histological normal liver radiotoxicity was observed. It is concluded that BPA-BNCT induced a significant remission of experimental colorectal tumor nodules in liver with no contributory liver toxicity.     

Abscopal effect of boron neutron capture therapy (BNCT): proof of principle in an experimental model of colon cancer

The aim of the present study was to evaluate, for the first time, the abscopal effect of boron neutron capture therapy (BNCT). Twenty-six BDIX rats were inoculated subcutaneously with 1 × 10⁶ DHD/K12/TRb syngeneic colon cancer cells in the right hind flank. Three weeks post-inoculation, the right leg of 12 rats bearing the tumor nodule was treated with BPA-BNCT (BPA-Boronophenylalanine) at the RA-3 nuclear reactor located in Buenos Aires, Argentina, at an absorbed dose of 7.5 Gy to skin as the dose-limiting tissue. The remaining group of 14 tumor-bearing rats were left untreated and used as control. Two weeks post-BNCT, 1 × 10⁶ DHD/K12/TRb cells were injected subcutaneously in the contralateral left hind flank of each of the 26 BDIX rats. Tumor volume in both legs was measured weekly for 7 weeks to determine response to BNCT in the right leg and to assess a potential influence of BNCT in the right leg on tumor development in the left leg. Within the BNCT group, a statistically significant reduction was observed in contralateral left tumor volume in animals whose right leg tumor responded to BNCT (post-treatment/pre-treatment tumor volume <1) versus animals who failed to respond (post/pre ≥1), i.e., 13 ± 15 vs 271 ± 128 mm³. In addition, a statistically significant reduction in contralateral left leg tumor volume was observed in BNCT-responsive animals (post/pre <1) vs untreated animals, i.e., 13 ± 15 vs 254 ± 251 mm³. The present study performed in a simple animal model provides proof of principle that the positive response of a tumor to BNCT is capable of inducing an abscopal effect.     

Biodistribution of sodium borocaptate (BSH) for boron neutron capture therapy (BNCT) in an oral cancer model

Boron neutron capture therapy (BNCT) is based on selective accumulation of ¹⁰B carriers in tumor followed by neutron irradiation. We previously proved the therapeutic success of BNCT mediated by the boron compounds boronophenylalanine and sodium decahydrodecaborate (GB-10) in the hamster cheek pouch oral cancer model. Based on the clinical relevance of the boron carrier sodium borocaptate (BSH) and the knowledge that the most effective way to optimize BNCT is to improve tumor boron targeting, the specific aim of this study was to perform biodistribution studies of BSH in the hamster cheek pouch oral cancer model and evaluate the feasibility of BNCT mediated by BSH at nuclear reactor RA-3. The general aim of these studies is to contribute to the knowledge of BNCT radiobiology and optimize BNCT for head and neck cancer. Sodium borocaptate (50 mg ¹⁰B/kg) was administered to tumor-bearing hamsters. Groups of 3–5 animals were killed humanely at nine time-points, 3–12 h post-administration. Samples of blood, tumor, precancerous pouch tissue, normal pouch tissue and other clinically relevant normal tissues were processed for boron measurement by optic emission spectroscopy. Tumor boron concentration peaked to therapeutically useful boron concentration values of 24–35 ppm. The boron concentration ratio tumor/normal pouch tissue ranged from 1.1 to 1.8. Pharmacokinetic curves showed that the optimum interval between BSH administration and neutron irradiation was 7–11 h. It is concluded that BNCT mediated by BSH at nuclear reactor RA-3 would be feasible.     

Therapeutic success of boron neutron capture therapy (BNCT) mediated by a chemically non-selective boron agent in an experimental model of oral cancer: a new paradigm in BNCT radiobiology

The hypothesis of boron neutron capture therapy (BNCT) research has been that the short-range, high-linear energy transfer radiation produced by the capture of thermal neutrons by (10)B will potentially control tumor and spare normal tissue only if the boron compound selectively targets tumor tissue within the treatment volume. In a previous in vivo study of low-dose BNCT mediated by GB-10 (Na(2)(10)B(10)H(10)) alone or combined with boronophenylalanine (BPA) in the hamster cheek pouch oral cancer model that was primarily designed to evaluate safety and feasibility, we showed therapeutic effects but no associated normal tissue radiotoxicity. In the present study, we evaluated the response of tumor, precancerous and normal tissue to high-dose BNCT mediated by GB-10 alone or combined with BPA. Despite the fact that GB-10 does not target hamster cheek pouch tumors selectively, GB-10-BNCT induced a 70% overall tumor response with no damage to normal tissue. (GB-10+BPA)-BNCT induced a 93% overall tumor response with no normal tissue radiotoxicity. Light microscope analysis showed that GB-10-BNCT selectively damages tumor blood vessels, sparing precancerous and normal tissue vessels. In this case, selective tumor lethality would thus result from selective blood vessel damage rather than from selective uptake of the boron compound.     

Boron neutron capture therapy (BNCT) for the treatment of spontaneous nasal planum squamous cell carcinoma in felines

Recently, Boron neutron capture therapy (BNCT) was successfully applied to treat experimental squamous cell carcinomas (SCC) of the hamster cheek pouch mucosa, with no damage to normal tissue. It was also shown that treating spontaneous nasal planum SCC in terminal feline patients with low dose BNCT is safe and feasible. In an extension of this work, the present study aimed at evaluation of the response of tumor and dose-limiting normal tissues to potentially therapeutic BNCT doses. Biodistribution studies with ¹⁰B-boronophenylalanine (BPA enriched in ¹⁰B) as a ¹⁰B carrier were performed on three felines that showed advanced nasal planum SCC without any standard therapeutic option. Following the biodistribution studies, BNCT mediated by ¹⁰BPA was done using the thermalized epithermal neutron beam at the RA-6 Nuclear Reactor. Follow-up included clinical evaluation, assessment of macroscopic tumor and normal tissue response and biopsies for histopathological analysis. The treated animals did not show any apparent radiation-induced toxicity. All three animals exhibited partial tumor control and an improvement in clinical condition. Enhanced therapeutic efficacy was associated with a high ¹⁰B content of the tumor and a small tumor size. BNCT is therefore believed to be potentially effective in the treatment of spontaneous SCC. However, improvement in targeting ¹⁰B into all tumor cells and delivering a sufficient dose at a greater depth are still required for the treatment of deep-seated, large tumors. Future studies are needed to evaluate the potential efficacy of the dual mode cellular (e.g. BPA-BNCT) and vascular (e.g. GB-10-BNCT) targeting protocol in a preclinical scenario, employing combinations of ¹⁰B compounds with different properties and complementary uptake mechanisms.     

Receptor-targeted liposomal delivery of boron-containing cholesterol mimics for boron neutron capture therapy (BNCT)

Liposomes have been a main focus of tumor-selective boron delivery strategies in boron neutron capture therapy (BNCT), a binary method for the treatment of cancer that is based on the nuclear reaction between boron atoms and low-energy thermal neutrons. Three novel carboranyl cholesterol derivatives were prepared as lipid bilayer components for the construction of nontargeted and receptor-targeted boronated liposomes for BNCT. A major structural feature of these novel boronated cholesterol mimics is the replacement of the B and the C ring of cholesterol with a carborane cluster. Computational analyses indicated that all three boronated compounds have structural features and physicochemical properties that are very similar to those of cholesterol. One of the synthesized boronated cholesterol mimics was stably incorporated into non-, folate receptor (FR)-, and vascular endothelial growth factor receptor-2 (VEGFR-2)-targeted liposomes. No major differences were found in appearance, size distribution, and lamellarity between conventional dipalmitoylphosphatidylcholine (DPPC)/cholesterol liposomes, nontargeted, and FR-targeted liposomal formulations of this carboranyl cholesterol derivative. FR-targeted boronated liposomes were taken up extensively in FR overexpressing KB cells in vitro, and the uptake was effectively blocked in the presence of free folate. In contrast, a boronated cholesterol mimic incorporated into nontargeted liposomes showed significantly lower cellular uptake. There was no apparent in vitro cytotoxicity in FR overexpressing KB cells and VEGFR-2 overexpressing 293/KDR cells when these were incubated with boronated FR- and (VEGFR-2)-targeted liposomes, respectively, although the former accumulated extensively in KB cells and the latter effectively interacted with VEGFR-2 by causing autophosphorylation and protecting 293/KDR cells from SLT (Shiga-like toxin)-VEGF cytotoxicity.     

"Sequential" boron neutron capture therapy (BNCT): a novel approach to BNCT for the treatment of oral cancer in the hamster cheek pouch model

In the present study the therapeutic effect and potential toxicity of the novel "Sequential" boron neutron capture therapy (Seq-BNCT) for the treatment of oral cancer was evaluated in the hamster cheek pouch model at the RA-3 Nuclear Reactor. Two groups of animals were treated with "Sequential" BNCT, i.e., BNCT mediated by boronophenylalanine (BPA) followed by BNCT mediated by sodium decahydrodecaborate (GB-10) either 24 h (Seq-24h-BNCT) or 48 h (Seq-48h-BNCT) later. In an additional group of animals, BPA and GB-10 were administered concomitantly [(BPA + GB-10)-BNCT]. The single-application BNCT was to the same total physical tumor dose as the "Sequential" BNCT treatments. At 28 days post-treatment, Seq-24h-BNCT and Seq-48h-BNCT induced, respectively, overall tumor responses of 95 ± 2% and 91 ± 3%, with no statistically significant differences between protocols. Overall response for the single treatment with (BPA + GB-10)-BNCT was 75 ± 5%, significantly lower than for Seq-BNCT. Both Seq-BNCT protocols and (BPA + GB-10)-BNCT induced reversible mucositis in the dose-limiting precancerous tissue around treated tumors, reaching Grade 3/4 mucositis in 47 ± 12% and 60 ± 22% of the animals, respectively. No normal tissue toxicity was associated with tumor response for any of the protocols. "Sequential" BNCT enhanced tumor response without an increase in mucositis in dose-limiting precancerous tissue.     

Boron neutron capture therapy for malignant melanoma: an experimental approach

Previous studies have shown that some thioamides, e.g., thiouracil, are incorporated as false precursors into melanin during its synthesis. If boronated analogs of the thioamides share this property, the melanin of melanotic melanomas offers a possibility for specific tumoural uptake and retention of boron as a basis for neutron capture therapy. We report on the synthesis of boronated 1H-1,2,4-triazole-3-thiol (B-TZT), boronated 5-carboxy-2-thiouracil (B-CTU), and boronated 5-diethylaminomethyl-2-thiouracil (B-DEAMTU) and the localization of these substances in melanotic melanomas transplanted to mice. The distribution in the mice was studied by boron neutron capture radiography. B-TZT and B-CTU showed the highest tumour:normal tissue concentration ratios, with tumour:liver ratios of about 4 and tumour:muscle ratios of about 14; B-DEAMTU showed corresponding ratios of 1.4 and 5, respectively. The absolute concentration of boron in the tumours, however, was more than three times higher in the mice injected with B-TZT, compared with B-CTU. The results suggest that B-TZT may be the most promising compound of the three tested with regard to possible therapy of melanotic melanomas.     

Boron neutron capture synovectomy (BNCS) as a potential therapy for rheumatoid arthritis: boron biodistribution study in a model of antigen-induced arthritis in rabbits

Boron neutron capture synovectomy (BNCS) is explored for the treatment of rheumatoid arthritis (RA). The aim of the present study was to perform boron biodistribution studies in a model of antigen-induced arthritis (AIA) in female New Zealand rabbits, with the boron carriers boronophenylalanine (BPA) and sodium decahydrodecaborate (GB-10) to assess the potential feasibility of BNCS for RA. Rabbits in chronic phase of AIA were used for biodistribution studies employing the following protocols: intra-articular (ia) (a) BPA-f 0.14 M (0.7 mg ¹⁰B), (b) GB-10 (5 mg ¹⁰B), (c) GB-10 (50 mg ¹⁰B) and intravenous (iv), (d) BPA-f 0.14 M (15.5 mg ¹⁰B/kg), (e) GB-10 (50 mg ¹⁰B/kg), and (f) BPA-f (15.5 mg ¹⁰B/kg) + GB-10 (50 mg ¹⁰B/kg). At different post-administration times (13–85 min for ia and 3 h for iv), samples of blood, pathological synovium (target tissue), cartilage, tendon, muscle, and skin were taken for boron measurement by inductively coupled plasma mass spectrometry. The intra-articular administration protocols at <40 min post-administration both for BPA-f and GB-10, and intravenous administration protocols for GB-10 and [GB-10 + BPA-f] exhibited therapeutically useful boron concentrations (>20 ppm) in the pathological synovium. Dosimetric estimations suggest that BNCS would be able to achieve a therapeutically useful dose in pathological synovium without exceeding the radiotolerance of normal tissues in the treatment volume, employing boron carriers approved for use in humans. Radiobiological in vivo studies will be necessary to determine the actual therapeutic efficacy of BNCS to treat RA in an experimental model.     

Synthesis and biological evaluation of boronated nucleosides for boron neutron capture therapy (BNCT) of cancer

Several N-3 substituted carboranyl Thd analogs were synthesized. These agents as well as some non-boronated nucleosides were evaluated in phosphoryl transfer assays with recombinant human TK1 and TK2. For some carboranyl thymidine analogs, TK1 phosphorylation rates approached 38% that of thymidine. Their in vitro cytotoxicty appeared to correlate with the TK1 levels in the tested cells. In some cases increased uptake in tumor cell nuclei compared with the surrounding cytoplasm was detected in vitro.     

A new approach for the synthesis of isonitrile carborane derivatives. Ligands for metal based boron neutron capture therapy (BNCT) and boron neutron capture synovectomy (BNCS) agents

A new approach for the synthesis of carborane isonitrile derivatives was developed. This approach involved the dehydration of both boron and carbon derived formamides using the Burgess reagent. The products, some of which were characterized by X-ray crystallography, can now be used as ligands for the synthesis of transition metal based boron neutron capture therapy and synovectomy agents and targeted radiopharmaceuticals.     

Tumor blood vessel "normalization" improves the therapeutic efficacy of boron neutron capture therapy (BNCT) in experimental oral cancer

We previously demonstrated the efficacy of BNCT mediated by boronophenylalanine (BPA) to treat tumors in a hamster cheek pouch model of oral cancer with no normal tissue radiotoxicity and moderate, albeit reversible, mucositis in precancerous tissue around treated tumors. It is known that boron targeting of the largest possible proportion of tumor cells contributes to the success of BNCT and that tumor blood vessel normalization improves drug delivery to the tumor. Within this context, the aim of the present study was to evaluate the effect of blood vessel normalization on the therapeutic efficacy and potential radiotoxicity of BNCT in the hamster cheek pouch model of oral cancer. Blood vessel normalization was induced by two doses of thalidomide in tumor-bearing hamsters on 2 consecutive days. All studies in thalidomide-treated animals were performed 48 h after the first dose of thalidomide, previously established as the window of normalization. Biodistribution studies were performed with BPA at a dose of 15.5 mg (10)B/kg in thalidomide-treated (Th+) and untreated (Th-) tumor-bearing hamsters. The effect of blood vessel normalization prior to BPA administration on the efficacy of BNCT was assessed in in vivo BNCT studies at the RA-3 Nuclear Reactor in tumor-bearing hamsters. Group I was treated with BPA-BNCT after treatment with thalidomide (Th+ BPA-BNCT). Group II was treated with BPA-BNCT alone (Th- BPA-BNCT). Group III was treated with the beam only after treatment with thalidomide (Th+ BO), and Group IV was treated with the beam only (Th- BO). Groups I and II were given the same dose of BPA (15.5 mg (10)B/kg), and all groups (I-IV) were exposed to the same neutron fluence. Two additional groups were treated with the beam only at a higher dose to exacerbate mucositis in precancerous tissue and to explore the potential direct protective effect of thalidomide on radiation-induced mucositis in a scenario of more severe toxicity, i.e. Group V (Th+ hdBO) and Group VI (Th- hdBO). The animals were followed for 28 days. Biodistribution studies revealed no statistically significant differences in gross boron content between Th+ and Th- animals. Overall tumor control (complete response + partial response) at 28 days post-treatment was significantly higher for Group I (Th+ BPA-BNCT) than for Group II (Th- BPA-BNCT): 84 ± 3% compared to 67 ± 5%. Pretreatment with thalidomide did not induce statistically significant changes in overall tumor control induced by the beam only, i.e. 15 ± 5% in Group III (Th+ BO) and 18 ± 5% in Group IV (Th- BO), or in overall tumor control induced by the high-dose beam only, i.e. 60 ± 7% in Group V (Th+ hdBO) and 47 ± 10% in Group VI (Th- hdBO). BPA-BNCT alone (Group II) induced mucositis in precancerous tissue that reached Grades 3-4 in 80% of the animals, whereas pretreatment with thalidomide (Group I) prevented mucositis Grades 3 and 4 completely. Beam-only Group III (Th+ BO) exhibited only Grade 1 mucositis in precancerous tissue, whereas 17% of the animals in beam-only Group IV (Th- BO) reached Grade 2 mucositis. High-dose beam-only group V (Th+ hdBO) exhibited only Grade 2 mucositis, whereas high-dose beam-only group VI (Th- hdBO) reached Grade 3 mucositis in 83% of the animals. In all cases mucositis in precancerous tissue was reversible. No normal tissue radiotoxicity was observed with any of the protocols. Pretreatment with thalidomide enhanced the therapeutic efficacy of BNCT and reduced precancerous tissue toxicity.     

Boron neutron capture therapy (BNCT) in Finland: Technological and physical prospects after 20 years of experiences

Boron Neutron Capture Therapy (BNCT) is a binary radiotherapy method developed to treat patients with certain malignant tumours. To date, over 300 treatments have been carried out at the Finnish BNCT facility in various on-going and past clinical trials. In this technical review, we discuss our research work in the field of medical physics to form the groundwork for the Finnish BNCT patient treatments, as well as the possibilities to further develop and optimize the method in the future. Accordingly, the following aspects are described: neutron sources, beam dosimetry, treatment planning, boron imaging and determination, and finally the possibilities to detect the efficacy and effects of BNCT on patients.     

Boron uptake in normal melanocytes and melanoma cells and boron biodistribution study in mice bearing B16F10 melanoma for boron neutron capture therapy

Information on (10)B distribution in normal tissues is crucial to any further development of boron neutron capture therapy (BNCT). The goal of this study was to investigate the in vitro and in vivo boron biodistribution in B16F10 murine melanoma and normal tissues as a model for human melanoma treatment by a simple and rapid colorimetric method, which was validated by HR-ICP-MS. The B16F10 melanoma cell line showed higher melanin content than human melanocytes, demonstrating a greater potential for boronophenylalanine uptake. The melanocytes showed a moderate viability decrease in the first few minutes after BNCT application, stabilizing after 75 min, whereas the B16F10 melanoma showed the greatest intracellular boron concentration at 150 min after application, indicating a different boron uptake of melanoma cells compared to normal melanocytes. Moreover, at this time, the increase in boron uptake in melanoma cells was approximately 1.6 times higher than that in normal melanocytes. The (10)B concentration in the blood of mice bearing B16F10 melanoma increased until 90 min after BNCT application and then decreased after 120 min, and remained low until the 240th minute. On the other hand, the (10)B concentration in tumors was increased from 90 min and maximal at 150 min after application, thus confirming the in vitro results. Therefore, the present in vitro and in vivo study of (10)B uptake in normal and tumor cells revealed important data that could enable BNCT to be possibly used as a treatment for melanoma, a chemoresistant cancer associated with high mortality.     

Boron determination in liver tissue by combining quantitative neutron capture radiography (QNCR) and histological analysis for BNCT treatment planning at the TRIGA Mainz

The typical primary malignancies of the liver are hepatocellular carcinoma and cholangiocarcinoma, whereas colorectal liver metastases are the most frequently occurring secondary tumors. In many cases, only palliative treatment is possible. Boron neutron capture therapy (BNCT) represents a technique that potentially destroys tumor tissue selectively by use of externally induced, locally confined secondary particle irradiation. In 2001 and 2003, BNCT was applied to two patients with colorectal liver metastases in Pavia, Italy. To scrutinize the rationale of BNCT, a clinical pilot study on patients with colorectal liver metastases was carried out at the University of Mainz. The distribution of the (10)B carrier (p-borono-phenylalanine) in the liver and its uptake in cancerous and tumor-free tissue were determined, focusing on a potential correlation between the uptake of p-borono-phenylalanine and the biological characteristics of cancerous tissue. Samples were analyzed using quantitative neutron capture radiography of cryosections combined with histological analysis. Methodological aspects of the combination of these techniques and results from four patients enrolled in the study are presented that indicate that the uptake of p-borono-phenylalanine strongly depends on the metabolic activity of cells.     

Improvement of the boron neutron capture therapy (BNCT) by the previous administration of the histone deacetylase inhibitor sodium butyrate for the treatment of thyroid carcinoma

We have shown that boron neutron capture therapy (BNCT) could be an alternative for the treatment of poorly differentiated thyroid carcinoma (PDTC). Histone deacetylase inhibitors (HDACI) like sodium butyrate (NaB) cause hyperacetylation of histone proteins and show capacity to increase the gamma irradiation effect. The purpose of these studies was to investigate the use of the NaB as a radiosensitizer of the BNCT for PDTC. Follicular thyroid carcinoma cells (WRO) and rat thyroid epithelial cells (FRTL-5) were incubated with 1 mM NaB and then treated with boronophenylalanine ¹⁰BPA (10 μg ¹⁰B ml^?1) + neutrons, or with 2, 4-bis (α,β-dihydroxyethyl)-deutero-porphyrin IX ¹⁰BOPP (10 μg¹⁰B ml^?1) + neutrons, or with a neutron beam alone. The cells were irradiated in the thermal column facility of the RA-3 reactor (flux = (1.0 ± 0.1) × 10¹⁰ n cm^?2 s^?1). Cell survival decreased as a function of the physical absorbed dose in both cell lines. Moreover, the addition of NaB decreased cell survival (p < 0.05) in WRO cells incubated with both boron compounds. NaB increased the percentage of necrotic and apoptotic cells in both BNCT groups (p < 0.05). An accumulation of cells in G2/M phase at 24 h was observed for all the irradiated groups and the addition of NaB increased this percentage. Biodistribution studies of BPA (350 mg kg^?1 body weight) 24 h after NaB injection were performed. The in vivo studies showed that NaB treatment increases the amount of boron in the tumor at 2-h post-BPA injection (p < 0.01). We conclude that NaB could be used as a radiosensitizer for the treatment of thyroid carcinoma by BNCT.     

Characterization of neutron beams for boron neutron capture therapy: in-air radiobiological dosimetry

The survival curves and the RBE for the dose components generated in boron neutron capture therapy (BNCT) were determined separately in neutron beams at Japan Research Reactor No. 4. The surviving fractions of V79 Chinese hamster cells with or without 10B were obtained using an epithermal neutron beam (ENB), a mixed thermal-epithermal neutron beam (TNB-1), and a thermal (TNB-2) neutron beam; these beams were used or are planned for use in BNCT clinical trials. The cell killing effect of the neutron beam in the presence or absence of 10B was highly dependent on the neutron beam used and depended on the epithermal and fast-neutron content of the beam. The RBEs of the boron capture reaction for ENB, TNB-1 and TNB-2 were 4.07 +/- 0.22, 2.98 +/- 0.16 and 1.42 +/- 0.07, respectively. The RBEs of the high-LET dose components based on the hydrogen recoils and the nitrogen capture reaction were 2.50 +/- 0.32, 2.34 +/- 0.30 and 2.17 +/- 0.28 for ENB, TNB-1 and TNB-2, respectively. The RBEs of the neutron and photon components were 1.22 +/- 0.16, 1.23 +/- 0.16, and 1.21 +/- 0.16 for ENB, TNB-1 and TNB-2, respectively. The approach to the experimental determination of RBEs outlined in this paper allows the RBE-weighted dose calculation for each dose component of the neutron beams and contributes to an accurate inter-beam comparison of the neutron beams at the different facilities employed in ongoing and planned BNCT clinical trials.     

Design and synthesis of boron containing monosaccharides by the hydroboration of d-glucal for use in boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is one of the radiotherapies that involves the use of boron-containing compounds for the treatment of cancer. Boron-10 (¹⁰B) containing compounds that can accumulate in tumor tissue are expected to be suitable agents for BNCT. We report herein on the design and synthesis of some new BNCT agents based on a d-glucose scaffold, since glycoconjugation has been recognized as a useful strategy for the specific targeting of tumors. To introduce a boryl group into a d-glucose scaffold, we focused on the hydroboration of d-glucal derivatives, which have a double bond between the C1 and C2 positions. It was hypothesized that a C–B bond could be introduced at the C2 position of d-glucose by the hydroboration of d-glucal derivatives and that the products could be stabilized by conversion to the corresponding boronic acid ester. To test this hypothesis, we prepared some 2-boryl-1,2-dideoxy-d-glucose derivatives as boron carriers and evaluated their cytotoxicity and cellular uptake activity to cancer cells, especially under hypoxic conditions.     

Epithermal neutron beams for clinical studies of boron neutron capture therapy: a dosimetric comparison of seven beams

A comparison of seven epithermal neutron beams used in clinical studies of boron neutron capture therapy (BNCT) in Sweden (Studsvik), Finland (Espoo), Czech Republic (ReZ), The Netherlands (Petten) and the U.S. (Brookhaven and Cambridge) was performed to facilitate sharing of preclinical and clinical results. The physical performance of each beam was measured using a common dosimetry method under conditions pertinent to brain irradiations. Neutron fluence and absorbed dose measurements were performed with activation foils and paired ionization chambers on the central axis both in air and in an ellipsoidal water phantom. The overall quality of each beam was assessed by figures of merit determined from the total weighted dose profiles that assumed the presence of boron in tissue. The in-air specific beam contamination from both fast neutrons and gamma rays ranged between 8 and 65 x 10(-11) cGy(w) cm2 for the different beams and the epithermal neutron flux intensities available at the patient position differed by more than a factor of 20 from 0.2-4.3 x 10(9) n cm(-2) s(-1). Percentage depth dose profiles measured in-phantom for the individual photon, thermal and fast-neutron dose components differed only subtly in shape between facilities. Assuming uptake characteristics consistent with the currently used boronated phenylalanine, all the epithermal beams exhibit a useful penetration of 8 cm or greater that is sufficient to irradiate a lesion seated at the brain midline. The performance of the existing facilities will benefit from the introduction of advanced compounds through improved beam penetrability. This could increase by as much as 2 cm for the purest of beams, although the beam intensities generally need to be increased to between 2-5 x 10(9) n cm(-2) s(-1) to maintain manageable irradiation times. These data provide the first consistent measurement of beam performance at the different centers and will enable a preliminary normalization of the calculated patient dosimetry.